Myasthenia gravis (MG) is an autoimmune disease commonly associated with immune disorders in thymoma. The role of thymus myoid cells (TMCs) in the pathogenesis of autoimmune diseases has attracted much attention. Therefore, the present study was designed to reveal the impact of TMCs on the pathophysiology of tumor-associated MG (TAMG). This study included clinical patients and healthy volunteers and validated the potential role of TMCs in TAMG progression using a TMCs-deficient mouse model. Correlative findings showed that TMCs deletion affected thymic architecture in MG patients, as evidenced by the expression of key myogenic factors as well as AChR and RyRs receptors in the thymus. Further experimental validation showed that TMCs deletion increased the levels of Th1 and Th17 cells, decreased the levels of Th2 and Treg cells, and altered the secretion of corresponding cytokines, including IL-2, IL-4, IL-17, IL-22, and TGF-β concentrations. Co-culture of CD4+ T cells with Thy0517 cells or CD4+ T cells with a myoblastoid cell line using the Transwell system demonstrated that deletion of TMC inhibited the differentiation of CD4+ T cells to Treg cells. In this study, we hypothesized that TMCs are involved in TAMG progression by regulating CD4+ T cell differentiation.